There are well in excess of 100 million vehicles licensed to operate on the highway of the United States. At any one time there are in excess of 100,000 of these that are wanted for investigation for any one of many reasons. They may have exceeded established speed limits, be operating with stolen license plates or have been at the scene of any one or more of many criminal activities. At the present time license numbers of vehicles wanted for investigation are distributed to local police who by reference to printed or computerized lists scan traffic visually as they encounter in the course of their normal duties. Their task is complicated by the fact that, on a busy multi-lane highway, some 176 cars can pass a given observation point each minute. As a result the probability of intercepting a wanted vehicle has been very low.
There have been many attempts to automate the surveillance by equipping each vehicle with a transponder device that responds to an interrogation signal by emitting a code that is unique to each vehicle in the environment. These interrogation responses were to be presented to an operator who no longer would rely on visual observation. These approaches have failed because of the high density of vehicles on highways, particularly in urban areas, and by the sheer volume of data that would be presented to the human observer. For example, on a major highway there can be many more than 90 vehicles within 1/2 mile of an interrogation device and in a multi-lane environment, many of them will be at the same or nearly the same range and at the same or nearly the same relative velocity with respect to the interrogation device. It is therefore not possible to discriminate them on the basis of two-way signal transit time (radar time) or on the basis of doppler frequency difference. The response codes become hopelessly interleaved and many vehicles are either not identified at all or are misidentified.